1. Field of the Invention
The present invention relates to the formation of a semiconductor thin film, and more particularly, to a method of forming a compound semiconductor thin film on a silicon substrate. The present invention is applied to a GaAs/Si substrate for GaAs devices, which substrate is produced by growing an epitaxial GaAs thin film on a silicon (Si) substrate.
2. Description of the Related Art
A compound semiconductor substrate, for example, gallium arsenide (GaAs) substrate is produced by growing a single-crystal (GaAs) lump (ingot) by the use of a pulling method (Czochralski method) or the like, and then cutting the lump. Such a GaAs substrate is expensive, easily broken, and has a relatively small size.
Recently, proposals have been made to produce GaAs/Si substrates corresponding to the GaAs substrate, to enlarge the wafer size, increase the mechanical strength and thermal conductivity, and reduce costs. In this case, a large lattice mismatch of about 4% exists between GaAs and Si, and this hinders the epitaxial growth of GaAs, and accordingly, a buffer layer having a strained layer superlattice (SLS) structure is inserted between the Si substrate and the GaAs layer to relax the lattice mismatch. The SLS structure buffer layer is compressed of InGaAs-GaAs, GaAsP-GaAs or InGaAs-GaAsP system as disclosed, for example, in N. El-Masry, et al. "Defect Reduction in GaAs Epilayers on Si Substrates Using Strained Layer Superlattices", Mat. Res. Symp. Proc. Vol. 91, '1987, Materials Research Society, pp. 99-103. Nevertheless, the formation of the SLS structure buffer layer is complicated.
Attempts have been made to grow the GaAs layer directly on the Si substrate by adopting a two-step growth sequence (process), as disclosed in, for example, M. AKIYAMA et al., "Growth of GaAs on Si and its Application to FETs and LEDs", Mat. Res. Soc. Symp. Proc. Vol. 67, 1986, pp. 53-64. In the two-step growth sequence, the Si substrate (wafer) is heated to about 900.degree. C. in a gas flow of H.sub.2 and AsH.sub.3 to clean the surface thereof, is cooled to 450.degree. C. or less and a first GaAs thin layer is deposited on the Si substrate by a methalorganic chemical vapor deposition (MOCVD) method. Subsequently, the Si substrate is reheated to a temperature of from 700.degree. to 750.degree. C., a conventional GaAs growth temperature for the MOCVD method, and then a second GaAs layer is epitaxially grown by the MOCVD method. The first GaAs thin layer has an amorphous or an unstable crystal structure upon deposition and is changed into an epitaxial GaAs thin layer by the annealing (heating) of the second GaAs layer growth.
In the GaAs epitaxial thin film formed directly on the Si substrate, however, crystal defects, such as a high density of dislocations and many etch pits, still occur, due to differences in the lattice constants and thermal expansion coefficients of Si and GaAs. These crystal defects cause problems in devices which are formed by using the GaAs epitaxial film, for example, a reduced electron mobility bringing a lower switching speed of a GaAs transistor, a large dispersion of threshold voltages of GaAs transistors in a wafer (i.e., the GaAs/Si substrate), and a shorter life of light emitting devices such as lasers and LED's.